A joint prosthetic component is usually anchored in the bone with a pin or a stem (anchorage component); see J. Bone Joint Surg., Vol. 21, pp. 269-288.
Various anchorage techniques are described in U.S. Pat. Nos. 2,934,065 and 2,718,288 and in French Pat. No. 1,278,359.
A hip joint is often replaced, e.g. by using a metal stem anchored in the medullary canal with bone cement; see J. Bone Joint Surg., Vol. 42 B, pp. 28-30.
The compatibility and the biomechanical strength of these bone cements is unsatisfactory, since the plastic materials serving as binding agents release parts to the surrounding tissue, which may cause harmful side reactions in the organism. In addition the reaction heat released during the hardening of the synthetic adhesive may damage the tissue. These and other reactions may cause a loosening of the prosthesis. Therefore attempts were made to provide cement-free anchorage systems, in order to have the prosthetic components anchored without the aid of the so-called bone cements see U.S. Pat. No. 3,605,123.
As a rule it is hereby necessary to enlarge the surface of the shaft of the prosthesis, e.g. by applying waved surfaces, saw teeth etc.; see e.g. German Pat. No. 837 294.
In the following attempts were made to modify the surface of the prosthetic shaft structurally in order to enlarge the surface and to thus allow bony ingrowth. Thus German Offenlegungsschrift No. 21 27 843 discloses a porous metal coating which is firmly bound to the basic body of the same material. With all these coatings it was found that bony ingrowth did occur only under specific conditions. However, reproducible results which would apply for all patients were not obtained with such prosthetic surfaces. It must be assumed that further factors are involved in securing bony ingrowth and prognostically accessible anchorage.
When examining the question in how far the individual osteoblast and/or the layer of osteoblasts may be induced by different morphological or chemical structures or substances to regenerate bones and/or to build up specific trabeculae, it was found that regarding the osteoblast a specific morphological sructure in the anchorage part strongly enhances bone regeneration, whereas with the layer of osteoblasts a different specific morphological structure caused the improved formation of supporting trabeculae. In addition, the specific embodiments (topography) of the prosthetic anchorage which are necessary in order to obtain optimal statical results must be taken into consideration in the overall construction. Moreover, the overall stress and the movement sequence of a joint in view of the design of the prosthesis which is decisive for the direction of force must also be taken into account.
From the criteria found here four dimensions of structuralization of implants resulted. In the following they are called structures of the first to the fourth order (structures I to IV). According to this definition the structure of the first order (structure I) corresponds to the external design of the prosthesis, i.e. the form of the anchorage part. The structure of the second order (structure II) represents a view of the surface (topography). With structure II, for example, specific surface designs, such as a waved surface or a saw teeth-like form of the prosthetic shaft, are designated. The surface designs of structure II are meant to enhance the mechanical anchorage. According to the present definition, the structure of the third order (structure III) represents the microstructure of the surfaces. It designates, for example, surface forms (e.g. small spherical particles) up to the mm-range. Finally, what is meant by the structure of the fourth order (structure IV) is the ultrastructure (up to a size of 30 .mu.m), which is essential for the present invention.
One embodiment of the above-mentioned structures I to III is described in German Pat. No. 27 30 004. This publication reveals the implant structure from the prosthesis' design (structure I) to the microstructure (structure III), which, for example, may be present in the form of a spherical coating. The projections on the surface of the basic body are meant to cause as improved interconnection of the osseous tissue in the interstices between the projections and thus a more resistant anchorage of the osseous tissue, preferably without a binder. The surface layer of this known implant, however, has the drawback, that it is not resorbable, and thus the osteocytes do not sufficiently adhere to the basic body. The biological and chemotactical activities (inducing of bone growth) are unsatisfactory with non-resorbable surfaces. Finally, no additions, such as hemostyptics, osteogenic substances, vaso-active substances and hormones acting on bone can be incorporated in the surface layer of German Pat. No. 27 30 004.
German Pat. No. 26 20 907 describes a coating of non-resorbable plastic material comprising spherical filing materials consisting of calcium phosphate, which is resorbable to a high degree. The non-resorbable plastic matrix has the disadvantage that the plastic material is abraded by the interfacial shearing action and that the wear particles are not resorbable, thus causing inflammations and possibly the loosening of the prosthesis.
A further considerable drawback of the coating according to German Pat. No. 26 20 907 consist in the fact that the ceramic particles stated to be resorbable suck themselves full with the non-resorbable plastic material, thereby leading to a further reduction in the resorbability of the coating. For this reason the osseous tissue around the prosthetic anchorage cannot grow deeply and rapidly onto the basic body (stem), this resulting in a loss of the stable fixation.
It is an object of the present invention to provide a fully resorbable coating composition for surgical implants.
It is a further object of the present invention to provide a fully resorbable coating composition for surgical implants which enables deep and rapid ingrowth of bone.
It is a further object of the present invention to provide an anchorage component for surgical implants with a fully or partly coated surface, wherein the coating is completely resorbable.
It is a further object of the present invention to provide an anchorage component for surgical implants with a fully or partly coated surface, the coating being completely resorbable and enabling deep and fast ingrowth of bone onto the basic body of the implant.
It is a further object of the present invention to provide an anchorage component for surgical implants with a fully or partly coated surface, said coating being completely resorbable, which anchorage component secures solid anchorage of the implant. It is still a further object of the present invention to provide a method for treating bone defects, which enables fast and deep ingrowth of the bone into the surface layer of the implant and which secures a firm anchorage of the implant.
The above and other objects and advantages of the present invention will be evident from the following description of the invention.